Addiction is costly to both the individuals and society, and relapses after current treatment methods are common. To significantly affect the prevalence of addiction and other reward-related disorders, it is imperative to understand the neural mechanisms underlying reward regulation. Dopamine deficits are well documented in patients with reward disorders. Dopamine is hypothesized to affect reward regulation by modulating response choice and the vigor with which the response is executed. However, no study to date has measured endogenous dopamine function in vivo in humans to directly examine how the dopamine system mediates response choice and response vigor. This research plan will use positron emission tomography (PET) to assess endogenous dopamine function and functional magnetic resonance imaging (fMRI) to identify the shared and differential neural correlates of response choice and response vigor. In addition, personality traits associated with addiction will be assessed. The goal of all aims is to develop a multilevel understanding of reward regulation through endogenous dopamine function, brain networks seen with fMRI, personality traits, response choice, and response vigor. The applicant's long-term goals are to identify deviations in the dopamine system leading to reward disorders and to develop effective interventions targeting specific aspects of dopamine function. This fellowship will help the applicant build an independent research program that leads to a productive career as an investigator of reward-driven behavior.